I re-read Frank Herbert's sci-fi masterpiece a few months back. It was a exciting today as it was when I first read it as a teenager. These rock formations reminded me of his descriptions of the deep desert of Arrakis, (Dune). These eroded formations are called tafoni. Thanks to Sampost for the id. Here is Wikipedia: Tafoni (singular: tafone) are small cave-like features found in granular rock such as sandstone, granite, and sandy-limestone with rounded entrances and smooth concave walls, often connected, adjacent, and/or networked. They often occur in groups that can riddle a hillside, cliff, or other rock formation. They can be found in all climate types, but are most abundant in intertidal areas and semi-arid and arid deserts. Currently favored explanations controlling their formation include salt weathering, differential cementation, structural variation in permeability, wetting-drying, and freezing-thawing cycles, variability in lithology, case hardening and core softening, and/or micro-climate changes and variation (i.e. moisture availability). Tafoni have also been called fretting, stonelace, stone lattice, honeycomb weathering, and alveolar weathering. Tafoni features have been found across the Earth, with dramatic forms found in the Jodhpur-Ajmer section of India's Thar Desert, Petra, Jordan, Coastal California and Australia, and even in the Arctic regions, and Antarctica (Paradise 2011).